It wasn't smart of them to test only for rabies. If the cat ingested a toxic substance, they'd be better off knowing if it is something they have inside their house. Especially with a child living on the premises. A vet or an animal control officer could have caught or sedated the cat, and caged it for testing. It's done all the time.
Sudden onset of that type of uncharacteristic behavior sounds far more like the cat was poisoned, seriously injured or partially electrocuted than it does anything else. Shooting the animal in the chest wasn't a solution, it was a failure of human intelligence.
I agree. If one of my otherwise sweet cats suddenly went psycho one day, I'd capture it (I know, not real easy with a crazy cat), and take it to the vet -- if the vet couldn't find anything, then lock it in a room for at least a day or two to see if it wears off.
And along with other suggestions as to possible causes (poison, electric shock, etc.), I'd like to add that the cat may have had a stroke, aneurism, or concussion.
It really is amazing how scary even a ten-pound housecat can be if they go into full-out aggressive mode. There's always a lot of talk about "dangerous dogs" or "killer breeds" and such, because a handful of people get injured or killed by dog attacks each year, but if housecats grew to the same size as a German Shepherd or whatever, the human deathtoll would be in the thousands. It'd be like having a cougar in the house.
I bet Horatio (David Caruso) would have figured out what caused the cat to go nuts.